Many psychologists assert that it has to do with a combination of trust and control. Doms typically desire a sense of control in the bedroom. They become aroused by the idea of being in control. And they feel a sense of love and appreciation by the the fact that their partner trusts them fully. Keep in mind that I said "sense of control" as opposed to "control". In most healthy BDSM relationships, the sub holds most of the control. And BDSM doesn't involve real pain or anything that's non-consensual; it's merely theatrics. Subs, conversely, gain pleasure by relinquishing control. In a world where one is constantly trying to control their life, their job, their bodies, their thoughts, giving up control for a moment can be quite relaxing. And they feel a sense of love by the knowledge that they can trust their partner fully.

But, from personal experience, BDSM is enjoyable because normal sex gets boring after a while. Gotta spice it up a bit on occasion...